Multi-pinhole System Research Articles

Performance evaluation of a novel multi-pinhole collimator for dopamine transporter SPECT

There is a tradeoff between spatial resolution and count sensitivity in SPECT with conventional collimators. Multi-pinhole (MPH) collimator technology has potential for concurrent improvement of resolution and sensitivity in clinical SPECT of ‘small’ organs. This study evaluated a novel MPH collimator specifically designed for dopamine transporter (DAT) SPECT with a triple-head SPECT camera. Count sensitivity was measured with a 99mTc point source placed on the lattice points of a 1 cm grid covering the whole field-of-view (FOV). Spatial resolution was assessed with a Derenzo type hot rod phantom. An anthropomorphic striatum phantom was scanned with total activity representative of a typical patient scan and different striatum-to-background activity concentration ratios. Recovery of striatum-to-background contrast was assessed by the contrast-recovery-coefficient. Measurements were repeated with double-head SPECT with fan-beam or low-energy-high-resolution-high-sensitivity (LEHRHS) collimators. A patient referred to DAT SPECT because of suspicion of Parkinson’s disease was scanned with both LEHRHS and MPH collimators after a single tracer injection. The axial MPH sensitivity profile was approximately symmetrical around its peak, although it was shifted 7 cm towards the patient to simplify positioning. Peak sensitivity of the triple-head MPH system in the center of the FOV was 620 cps MBq−1 compared to 225 cps MBq−1 for the double-head fan-beam system. Sensitivity of the MPH system decreased towards the edges of the FOV. The full width of the sensitivity profile at 200 cps MBq−1 was 21 cm transaxially and 11 cm axially. In MPH SPECT of the Derenzo phantom all rods with ≥ 5 mm diameter were clearly visible. MPH SPECT improved striatal contrast recovery by ≥ 20% compared to fan-beam SPECT. The patient scan demonstrated good image quality of MPH SPECT with almost PET-like delineation of putamen and caudate nucleus. SPECT with dedicated MPH collimators provides considerable improvement of the resolution-sensitivity tradeoff in DAT SPECT compared to SPECT with fan-beam or LEHRHS collimators.

Simulations of a Multi-Pinhole SPECT Collimator for Clinical Dopamine Transporter (DAT) Imaging.

SPECT imaging of the dopamine transporter (DAT) is used for diagnosis and monitoring progression of Parkinson's Disease (PD), and differentiation of PD from other neurological disorders. The diagnosis is based on the DAT binding in the caudate and putamen structures in the striatum. We previously proposed a relatively inexpensive method to improve the detection and quantification of these structures for dual-head SPECT by replacing one of the fan-beam collimators with a specially designed multi-pinhole (MPH) collimator. In this work, we developed a realistic model of the proposed MPH system using the GATE simulation package and verified the geometry with an analytic simulator. Point source projections from these simulations closely matched confirming the accuracy of the pinhole geometries. The reconstruction of a hot-rod phantom showed that 4.8 mm resolution is achievable. The reconstructions of the XCAT brain phantom showed clear separation of the putamen and caudate, which is expected to improve the quantification of DAT imaging and PD diagnosis. Using this GATE model, point spread functions modeling physical factors will be generated for use in reconstruction. Also, further improvements in geometry are being investigated to increase the sensitivity of this base system while maintaining a target spatial resolution of 4.5-5 mm.

An extension to artifact-free projection overlaps.

In multipinhole single photon emission computed tomography, the overlapping of projections has been used to increase sensitivity. Avoiding artifacts in the reconstructed image associated with projection overlaps (multiplexing) is a critical issue. In our previous report, two types of artifact-free projection overlaps, i.e., projection overlaps that do not lead to artifacts in the reconstructed image, were formally defined and proved, and were validated via simulations. In this work, a new proposition is introduced to extend the previously defined type-II artifact-free projection overlaps so that a broader range of artifact-free overlaps is accommodated. One practical purpose of the new extension is to design a baffle window multipinhole system with artifact-free projection overlaps. First, the extended type-II artifact-free overlap was theoretically defined and proved. The new proposition accommodates the situation where the extended type-II artifact-free projection overlaps can be produced with incorrectly reconstructed portions in the reconstructed image. Next, to validate the theory, the extended-type-II artifact-free overlaps were employed in designing the multiplexing multipinhole spiral orbit imaging systems with a baffle window. Numerical validations were performed via simulations, where the corresponding 1-pinhole nonmultiplexing reconstruction results were used as the benchmark for artifact-free reconstructions. The mean square error (MSE) was the metric used for comparisons of noise-free reconstructed images. Noisy reconstructions were also performed as part of the validations. Simulation results show that for noise-free reconstructions, the MSEs of the reconstructed images of the artifact-free multiplexing systems are very similar to those of the corresponding 1-pinhole systems. No artifacts were observed in the reconstructed images. Therefore, the testing results for artifact-free multiplexing systems designed using the extended type-II artifact-free overlaps numerically validated the developed theory. First, the extension itself is of theoretical importance because it broadens the selection range for optimizing multiplexing multipinhole designs. Second, the extension has an immediate application: using a baffle window to design a special spiral orbit multipinhole imaging system with projection overlaps in the orbit axial direction. Such an artifact-free baffle window design makes it possible for us to image any axial portion of interest of a long object with projection overlaps to increase sensitivity.

AN EFFICIENT SENSITIVITY CALCULATION OF TILTED APERTURES FOR PRECLINICAL MULTI-PINHOLE SPECT

Sensitivity performance is one of the key issues in multi-pinhole collimator design for small animal pinhole Single photon emission computed tomography (SPECT). Currently, there are two approaches in predicting sensitivity performance: analytical formula and Monte Carlo (MC) simulation. Analytical formula offers fast computation, while MC simulation provides better modeling of various physical effects and generates more accurate sensitivity prediction. Tilted pinhole apertures become popular in modern system design, because they can avoid projection overlapping and increase field-of-view (FOV) compared to traditional multiple pinholes. However, conventional analytical formula for sensitivity prediction cannot be directly applied to tilted apertures. In this research, we present a modified analytical formula to predict the sensitivity performance by considering tilted and translated pinhole apertures in a multi-pinhole collimation design. The modification is based on a construction of a virtual object plane which is parallel to the aperture plane in the tilted pinhole. Since the new formula is derived in the vector domain, it can be readily integrated to computer-aided-design software to greatly simplify the collimator design optimization. The results show that the modified formula generates sensitivity prediction similar to that from the MC simulation for a multi-pinhole system with tilted pinholes. When larger tilted pinholes are used to increase FOV, the formula can also accurately generate sensitivity prediction with a slightly reduced peak value. For a tilted pinhole aperture up to 40°, the simulation results indicate that the conventional analytical formula may overestimate as much as 25% sensitivity.

Onboard functional and molecular imaging: A design investigation for robotic multipinhole SPECT

Onboard imaging-currently performed primarily by x-ray transmission modalities-is essential in modern radiation therapy. As radiation therapy moves toward personalized medicine, molecular imaging, which views individual gene expression, may also be important onboard. Nuclear medicine methods, such as single photon emission computed tomography (SPECT), are premier modalities for molecular imaging. The purpose of this study is to investigate a robotic multipinhole approach to onboard SPECT. Computer-aided design (CAD) studies were performed to assess the feasibility of maneuvering a robotic SPECT system about a patient in position for radiation therapy. In order to obtain fast, high-quality SPECT images, a 49-pinhole SPECT camera was designed which provides high sensitivity to photons emitted from an imaging region of interest. This multipinhole system was investigated by computer-simulation studies. Seventeen hot spots 10 and 7 mm in diameter were placed in the breast region of a supine female phantom. Hot spot activity concentration was six times that of background. For the 49-pinhole camera and a reference, more conventional, broad field-of-view (FOV) SPECT system, projection data were computer simulated for 4-min scans and SPECT images were reconstructed. Hot-spot localization was evaluated using a nonprewhitening forced-choice numerical observer. The CAD simulation studies found that robots could maneuver SPECT cameras about patients in position for radiation therapy. In the imaging studies, most hot spots were apparent in the 49-pinhole images. Average localization errors for 10-mm- and 7-mm-diameter hot spots were 0.4 and 1.7 mm, respectively, for the 49-pinhole system, and 3.1 and 5.7 mm, respectively, for the reference broad-FOV system. A robot could maneuver a multipinhole SPECT system about a patient in position for radiation therapy. The system could provide onboard functional and molecular imaging with 4-min scan times.

Performance Evaluation of Stationary and Semi-Stationary Acquisition with a Non-Stationary Small Animal Multi-Pinhole SPECT System

Step-and-shoot mode with many angular steps results in long frame duration limiting the capability of single-photon emission computed tomography (SPECT) for fast dynamic scans. The present study evaluates acquisition with reduced angular sampling for fast imaging in preclinical research with the nanoSPECT/CTplus four-head multi-pinhole system. Measurements with line sources, homogeneity phantoms and a Jaszczak phantom filled with (99m)Tc or (123)I were performed to evaluate the 'stationary' and 'semi-stationary' acquisition mode (one or two detector positions, respectively) with respect to spatial resolution, quantification, noise properties and image artefacts. An in vivo mouse study was performed with (99m)Tc-MAG3. The fast acquisition modes resulted in only minor degradation of spatial resolution and quantification accuracy. Statistical noise in reconstructed images was significantly reduced compared to conventional SPECT, particularly at low count statistics. Stationary acquisition resulted in streak artefacts and spatial distortion. The semi-stationary acquisition mode of the nanoSPECT/CTplus allows fast dynamic SPECT with tolerable loss of image quality.

Image Reconstructions from Limit Views and Angle Coverage Data for a Stationary Multi-Pinhole SPECT System

The goal of this paper is to investigate different reconstruction methods for solving the limited angle problem in reconstructing the projection data from a stationary multi-pinhole system based on a three-head clinical single photon emission computed tomography (SPECT) camera. Three iterative reconstruction approaches were studied including maximum likelihood expectation maximization (MLEM), maximum a posteriori expectation maximization (MAPEM) with a smoothing prior, and an alternating optimization schemes from MLEM and total variation (TV) optimization. A three-headed multipinhole SPECT was simulated with apertures of nine 1-mm pinholes each, and covered scanning volume of 6-mm diameter. The reconstructions were optimized for various iterations based on visual inspections, and finally 20 iterations were applied for each method. For both MLEM-TV and MAPEM-TV, various initial reconstructions before TV optimization were studied. The smoothing parameter for MAPEM and the gradient descent constant for TV were also investigated through visual comparison. The preliminary results showed the 3 reconstruction methods generated compatible images, and can restore the images from projection data suffering limited angular sampling. However, MLEM was noisy for low-count and highly limited angle data, and thus suitable smoothing in MAPEM alleviated this problem. Initial reconstructions were necessary for better edge enhancement in TV. The conclusion is that TV might be potential in producing more edge-enhanced images if all parameters were optimized.

Development of multi-pinhole collimator for large imaging area with high spatial resolution

A multi-pinhole collimator system, which is effective for enlarging imaging area with high spatial resolution, has been demonstrated on Cold Neutron Radiograph Facility at JRR-3M in JAEA. The multi-pinhole collimator system consists of a multi-pinhole aperture and an overlap-cutter. On neutron beam from the multi-pinhole aperture, neutrons from each pinhole overlap at the sample position due to beam divergence. In the overlap area, sample images from each pinhole are obtained at a detector and these images look like split images. The overlap-cutter works to cut neutrons contribute to overlap at sample position and enable us to get an image without split. Imaging area was enlarged and sharp image without split was successfully obtained by using the multi-pinhole collimator system.

Modeling of the Point Spread Function by Numerical Calculations in Single-Pinhole and Multipinhole SPECT Reconstruction

In conventional reconstruction of single photon emission computed tomography (SPECT) data acquired with a single-pinhole or multipinhole system, the point spread function (PSF) may be either approximated by some analytical equations or substituted by the sensitivity function, which is the integral of the PSF. We have developed a method to numerically calculate the PSF for a pinhole system in order to improve image resolution over a sensitivity-function-based method. The method calculates the probability of photon penetration through the pinhole edges using a ray-tracing approach. To calculate the transmission by the collimator plate along each ray, we trace the ray through the collimator by analytical calculations. The PSF is calculated for only one detector angle, and a Gaussian rotator is used to rotate the image grid for other detector angles in the iterative reconstruction. To evaluate our method, we measured the sensitivities of four keel-edged single-pinhole plates and scanned an ultramicro Derenzo phantom on a single-pinhole system and a five-pinhole system and performed two mouse bone scans on the five-pinhole system using the 140 keV photons of Tc-99m. The numerical calculations of sensitivities for the single-pinhole plates agreed well with the measurements. Results for both types of data scans showed that modeling of the PSF improved image resolution. In conclusion, we found that modeling of the PSF by numerical calculations increases the resolution of reconstruction for single-pinhole and multipinhole SPECT imaging.

A Skew-Slit Collimator for Small-Animal SPECT

The main objective of this work was the development of a skew-slit collimator for small-animal SPECT to replace the state-of-the-art multipinhole collimator. A pinhole forms a cone-beam imaging geometry. If the collimator rotates around the object in a circular orbit, the projection measurements acquired by the cone-beam imaging geometry are incomplete and not enough for artifact-free image reconstruction. The severity of the artifact is proportional to the cone angle of the pinhole in the direction of the axis of rotation. Multipinhole geometry can greatly reduce the data-insufficiency artifacts; however, image magnification is sacrificed. By transforming a pinhole into a pair of skewed slits, we are able to use a large cone angle in the transaxial direction to increase image magnification and a small cone angle in the direction of the axis of rotation to reduce data-insufficiency artifacts. This transformation is achieved by placing a vertical slit (i.e., the slit is parallel to the axis of rotation) close to the object and placing a horizontal slit farther out. Similar to the multipinhole collimator, we also propose a multiple-skew-slit collimator that has a single vertical slit and several horizontal slits. Computer simulations were performed to verify the working principle of the skew-slit collimator. A prototype multiple-skew-slit collimator was fabricated, and phantom experiments were performed on a SPECT system. The smallest channels (of 0.75-mm diameter) in the phantom were clearly separated in the reconstructed image. This study suggests a novel technology in small-animal SPECT to replace the multipinhole collimator, resulting in significantly reduced image artifacts and increased transaxial resolution. A physical multiple-skew-slit system was built and tested. Compared with the multipinhole system, the multiple-skew-slit system has a larger image magnification in the transaxial direction and thus has better image resolution.

Optimal number of pinholes in multi-pinhole SPECT for mouse brain imaging—a simulation study

This study simulates a multi-pinhole single-photon emission computed tomography (SPECT) system using the Monte Carlo method, and investigates different multi-pinhole designs for quantitative mouse brain imaging. Prior approaches investigating multi-pinhole SPECT were not often optimal, as the number and geometrical arrangement of pinholes were usually chosen empirically. The present study seeks to optimize the number of pinholes for a given pinhole arrangement, and also for the specific application of quantitative neuroreceptor binding in the mouse brain. An analytical Monte Carlo simulation based method was used to generate the projection data for various count levels. A three-dimensional ordered-subsets expectation-maximization algorithm was developed and used to reconstruct the images, incorporating a realistic pinhole model for resolution recovery and noise reduction. Although artefacts arising from overlapping projections could be a major problem in multi-pinhole reconstruction, the cold-rod phantom study showed minimal loss of spatial resolution in multi-pinhole systems, compared to a single-pinhole system with the same pinhole diameter. A quantitative study of neuroreceptor binding sites using a mouse brain phantom and low activity (37 MBq) showed that the multi-pinhole system outperformed the single-pinhole system by maintaining the mean and lowering the variance in the measured uptake ratio. Multi-pinhole collimation can be used to reduce the injected dose and thereby reduce the radiation exposure to the animal. Results also suggest that the nine-pinhole configuration shown in this paper is a good choice for mouse brain imaging.